A well fitting dressage saddle should "put you" in a natural balanced and up-right position.
Your seat should naturally rest in the centre of the saddle (the deepest point) with approximately one hand-width room at the pommel and also at the cantle.
The saddle flap should gently contour the shape and length of your leg, "providing" your knees with the support of a correctly placed knee-roll (without restricting). Your knees would be bent slightly, so your calf/heel-line would be naturally positioned under your hip without you using any force.
The result would be your "up-right position" or dressage seat.
Of course - what I find important is the design of the seat itself. We are all built differently. Some have wider hips than others (referring to the skelettal make-up....not the "bulges".....;-) )
Peter Horobin (saddler in Victoria)has released quite an interesting additorial somewhere (can't remember where I read it) - he even talks about a design especially for the female pelvis!
I shall look for the reference and post it.
Can anyone else help?